Efforts in the development of poxvirus-based vaccines to prevent HIV-I have been focused on the SIV model: A protocol designed to obtain preclinical data in the pathogenic SIV251 macaque model is ongoing. The protocol was designed to investigate the modulatory effect of interleukines such as IL-2 and IL-12 on the immunogenicity of a NYVAC- SIV gag, pol, and env vaccine. The adjuvant effect of NYVAC-IL-2 and NYVAC-IL-12, at the time of immunization, was evidenced by changes in the systemic immune responses against viral antigens in vaccinees. Higher cellular immune responses, at the expenses of humoral immune response, were measured in animals exposed to IL-12 at the time of immunization. The immunological modulation induced by IL-2 and IL-12 , however, did not correlate with the ability of the animals to control the acute viremia.The long term outcome of viral challenge demonstrated that,upon intrarectal challenge, half of the animals cleared viral infection and the remaining half become infected as the control animals. Upon intravenous challenge, all the animals became infected but half of them controlled viremia efficiently and did not progress to disease. These cohorts provide a model to study the contribution of different viral subtypes (with different coreceptor usage) in establishing a persistent infection, and disease induction. Because analysis of the classical immunological parameter in the blood of immunized animals cytotoxic lymphocyte activity, T-cell responses upon Ag stimulation, neutralizing Ab)did not provide correlate of immunity, we sought to investigate novel approaches to assess the importance of cell mediated immunity in protection. The lack of availability of syngenetic animals coupled with the difficulty of ablating completely CD8+ cells in vivo, prompted us to investigate a histoculture system developed in Dr. Margolis' laboratory (NICHD/LTPB). Lymph node histocultures from vaccinated/protected and naive animals were tested in vitro for their susceptibility to infection by the viral challenge. All lymph node from vaccinated/protected animals demonstrated resistence to viral infection which could be relieved in some cases by the addition of Ab against the MHC class 1,suggesting a role of cell mediated immunity in protection. The same antibody had no effect on the infectability of lymph node from naive animals. This in vitro histoculture system provides an in vitro tool to study not only immune correlates but also selection of viral genotype/phenotype induced by the host response. Chimpanzee model: A protocol designed to compare the immunogenicity and efficacy of NYVAC and ALVAC HIV vaccines in chimpanzees was initiated in 1995. The animals have been immunized four times with recombinant poxvirus vectors carrying the gag, pol, and env genes. The plan is to challenge these animals intrarectally with the chimpanzee-adapted HIV-IC499 isolate, which is being tested now for infectivity.